Why do dogs have such short lifespans?Lifespan in general is determined by trade-offs between survival and reproduction. Wolves, the ancestors of dogs, can live 15-20 years*, roughly twice as long as comparable-sized dogs. They start breeding in the wild no younger than 2 years. They need to form pairs and establish a territory before breeding. Older wolves will often have help raising their pups from older juveniles who have not managed to mate or find territories. In contrast, most dogs can breed from 6-12 months of age, and they don't benefit from having territories, pair bonds or packs. Whereas wolves breed until they die, dog breeders will usually retire older females. So the whole life history of dogs is shifted to more of a "live-fast-die-young" style compared with wolves.
On top of that, artificial selection and inbreeding have created huge problems for dogs. Here's what dogs look like, when they're not bred to conform to human expectations: